1. Field of the Invention
The invention relates to optical communications system having waveguides or waveguide devices with tunable properties.
2. Discussion of the Related Art
As the prevalence of optical communications systems has increased, numerous techniques for modifying and/or controlling propagation of light through waveguides have been developed. Such techniques have included incorporation of photosensitive materials into the core of single mode optical fibers, to allow formation of periodic refractive index modulations. Such modulations enabled fiber Bragg gratings (FBG) as well as long period gratings (LPG), which have become widely used for a variety of applications, including reflection of selected frequency bands and gain flattening. Continued research led to so-called microstructured fiber, in which the fiber contains axially oriented elementsxe2x80x94typically capillary air holesxe2x80x94that provide a variety of useful properties such as photonic crystal characteristics, supercontinuum generation, and soliton generation. (See, e.g., B. J. Eggleton et al., xe2x80x9cCladding-Mode-Resonances in Air-Silica Microstructure Optical Fibers,xe2x80x9d Journal of Lightwave Technology, Vol. 18, No. 8 (2000);.J. C. Knight et al., xe2x80x9cAnomalous Dispersion in Photonic Crystal Fiber,xe2x80x9d IEEE Photonics Technology Letters, Vol. 12, No. 7 (2000); J. Ranka et al., xe2x80x9cVisible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,xe2x80x9d Optics Letters, Vol. 25, No. 1 (2000); and U.S. Pat. Nos. 5,907,652 and 6,097,870.) Such microstructured fiber also allows adjustment of effective refractive index profiles to attain, for example, decoupling of interior cladding modes from the influence of the material surrounding the fiber. Combinations of microstructured fiber with in-fiber gratings have also shown interesting results. (See, e.g., B. J. Eggleton et al., xe2x80x9cGrating Resonances in Air-Silica Microstructured Optical Fibers,xe2x80x9d Optics Letters, Vol. 24, No. 21, 1460 (1999); P. S. Westbrook et al., xe2x80x9cCladding-Mode Resonances in Hybrid Polymer-Silica Microstructured Optical Fiber Gratings,xe2x80x9d IEEE Phot. Tech. Lett., Vol. 12, No. 5, 495 (2000).)
More recent efforts have focused on attaining real-time tunability of the properties of gratings and/or microstructured fiber. For example, in J. A. Rogers et al., xe2x80x9cTemperature stabilised operation of tunable fibre grating devices that use distributed on-fiber thin film heaters,xe2x80x9d Electronics Letters, Vol. 35, No. 23, 2052 (1999), the authors describe a technique for thermally tuning the properties of fiber Bragg gratings or long period gratings. Specifically, a thin-film resistive heater is formed on the exterior of the fiber, and electrical control is used to tune and stabilize the grating properties. In Jeong et al., xe2x80x9cElectrically Controllable Long-Period Liquid Crystal Fiber Gratings,xe2x80x9d IEEE Phot. Tech. Lett., Vol. 12, No. 5, 519 (2000), the authors describe a fiber having a liquid crystal-filled core. A combed electrode, i.e., an electrode having periodic gaps, is used to selectively align the liquid crystals at the periodic distance of the electrode. The result is essentially a long period grating capable of being turned on and off (see FIG. 2 of the paper). In another approach, reflected in K. Takizawa et al., xe2x80x9cPolarization-independent optical fiber modulator by use of polymer-dispersed liquid crystals,xe2x80x9d Applied Optics, Vol. 37, No. 15, 3181 (1998), a ferrule, which has polymer-dispersed liquid crystals therein, is placed between two fibers to provide adjustable modulation of a propagating signal.
A variety of references report use of a second material either within a microstructured fiber or surrounding a fiber, where the second material is capable of undergoing a bulk refractive index change in response to external stimuli, e.g., heat. For example, in U.S. Pat. No. 6,058,226 to Starodubov, a fiber with a LPG therein is surrounded by a second material that undergoes a bulk index change in response to applied or encountered external stimuli. The resulting changes in the bulk refractive index of this second material alters the propagation and coupling of the core/cladding modes. U.S. Pat. No. 5,361,320 to Liu et al. discloses a fiber having a liquid crystal core or cladding, similar to what is disclosed in Jeong et al., supra. Liu discloses adjusting the orientation of all the liquid crystals, by electrical control, to provide a bulk index change in the material, this change altering the properties of the fiber core or cladding. In A. A. Abramov et al., xe2x80x9cElectrically Tunable Efficient Broad-Band Fiber Filter,xe2x80x9d IEEE Phot. Tech. Lett., Vol. 11, No. 4, 445 (1999), a microstructured fiber having a long period grating formed-therein is imbibed with, or surrounded by, a polymer having a temperature-sensitive-index of refraction. A resistive heating film is formed on the exterior of the fiber, and allows tuning of the bulk refractive index of the polymer. By changing the index of the polymer, the properties of the LPG, e.g., the resonance wavelength, can be controlled. Co-assigned U.S. Pat. No. 6,111,999 to Espindola et al. uses a similar approach. Specifically, a fiber having a grating written therein is provided with one or more variable refractive index regions. These regions contain a material having an adjustable bulk refractive index, such that adjusting the index of the regions provides a desired change in the properties of the grating.
While these numerous approaches to tunable and/or microstructured fiber and fiber gratings exist, further improvements and enhancements are always desired.
The invention provides a unique waveguide structure in which the waveguide contains individual scattering elements that are capable of being tuned to provide local refractive index variations, e.g., on a micron scalexe2x80x94which is on the order of wavelengths typically used for communication system. (Micron scale indicates that the individual elements provide tunable local index changes covering a distance of 0.1 to 10 xcexcm.) For example, for a system containing a source that launches one or more wavelengths into the waveguide, the scattering elements will have a size typically ranging from 0.1 to 10 times such wavelengths, typically 0.3 to 3 times such wavelengths. (Generally, the tunable local index changes are made on a length scale that provides the maximum scattering effect at the signal system""s wavelength of interest.)
According to the invention, the waveguide contains a core region, a cladding region, and a solid or liquid material having the tunable scattering elements dispersed therein, where the material is disposed within the core and/or cladding regions, and/or on the exterior of the cladding region. Useful scattering elements include, for example, liquid crystals dispersed in a polymer (polymer-dispersed liquid crystalsxe2x80x94PDLC) or electrophoretic particles dispersed in a liquid medium. (As used herein, a material having tunable scattering elements dispersed therein indicates, for example, any mixture, dispersion, suspension, solution, etc. that provides a material with distinct tunable elements or regions that provide local variation in refractive index.)
As noted above, several groups have explored the use in waveguides of materials capable of having their refractive index varied by external controls. However, these approaches, reflected for example in U.S. Pat. No. 6,058,226, U.S. Pat. No. 5,361,320, A. A. Abrambv et al., supra, and U.S. Pat. No. 6,111,999, rely on changing the bulk refractive index of the entire region of the waveguide in which the material is located. By contrast, the invention uses individual scattering elements that provide tunable local variations in the refractive index, such that the nature of the waveguide""s scattering cross-section can be adjusted. For example, it is possible to tune the scattering elements such that their refractive index (as encountered by propagating light) is substantially matched to the surrounding material in which they are dispersed. In such a case, the scattering cross-section is low. It is also possible to tune the scattering elements such that their refractive index is different from the surrounding material. In this case, the elements will induce scattering of the propagating light, which, as discussed below, provides some desirable effects.
One embodiment, which illustrates these features of the invention, is shown in FIGS. 5A and 5B. The waveguide is a microstructured fiber having six capillary air holes 58 surrounding a germanium-doped silica core region 52, in which a long period grating 56 is formed. The air holes are filled with a polymer-dispersed liquid crystal precursor and cured to induce formation of PDLC 60, i.e., phase separation and LC droplet formation. A thin metal film 62 is deposited on one side of the fiber, with wire leads attached to the film. With no current applied, the local refractive index profile within the LC droplets varies significantly with respect to the surrounding polymer, i.e., the LC are at a high scattering state, which causes the core mode/cladding mode coupling (induced by the LPG) to be somewhat inhibited. If a current is applied to the metal film to bring the fiber temperature to, the transition point at which the LC substantially match the refractive index of the surrounding polymer, i.e., the LC are at a low scattering state, the core mode/cladding mode coupling increases such that the LPG effects are increased.